STM observation of charge stripe in metallic phase of $\alpha -(\mathrm{BEDT-TTF}{)}_2{\mathrm{I}}_3$

We performed scanning tunneling microscopy at room temperature on the conducting surface of $\alpha -(\mathrm{BEDT-TTF}{)}_2{\mathrm{I}}_3$, which undergoes charge ordering below $T_{\mathrm{CO}}=135\mathrm{K}$. An $\alpha -\text{type}$ BEDT-TTF molecular arrangement was clearly observed. We observed a charge stripe structure with some inhomogeneity at room temperature. The horizontal charge stripe developed in most of the observed area. However, the coherence was restricted by the presence of charge uniform regions. The dimerization was found only between $A$ and $A^{\prime }$. The charge disproportionation state developed on the donor arrangement, which differs from that in the charge ordered state below $T_{\mathrm{CO}}$. A certain amount of stripe structure inhomogeneity is expected to be caused by the absence of the dimer of $B$ and $C$ BEDT-TTF molecules. We also found a local diagonal stripe that formed the boundary between two types of horizontal stripes.

Figure 1

Schema of the three types of charge order: (a) horizontal stripe, (b) vertical stripe, and (c) diagonal stripe. The circles represent the BEDT-TTF molecules and their diameters express the charge amount on the BEDT-TTF molecules.

Figure 2

(a) Crystal structure of $\alpha$-(BEDT-TTF)${}_2{\mathrm{I}}_3$ viewed from the $b$ axis. (b) BEDT-TTF arrangement of this salt. Column I consists of BEDT-TTF labeled $A$ and $A^{\prime }$. These are crystallographically equivalent. Column II consists of BEDT-TTF labeled $B$ and $C$.

Figure 3

Current image of an $a\text{−}b$ plane at bias voltage $V=50$ mV and tunneling current $I=50$ pA. A white rectangle indicates the unit cell of the BEDT-TTF molecule arrangement in the image.

Figure 4

(a) STM image with the same bias voltage and tunneling current. (b) Line profiles along the columns. The arrow on the left in (a) represents the direction of the horizontal axis of (b). Arrows in (b) indicate charge rich molecules. Gray rectangles in (a) and (b) indicate the charge uniform part.

Figure 5

(a) STM image around column I developing twofold periodicity with charge rich $A^{\prime }$ molecules. (b) Line profiles in (a). The arrow with the broken line represents the direction of the horizontal axis of (b). The arrows in (b) indicate charge rich molecules.

Figure 6

(a) Schema of domain structure. The domain boundaries between stripe structures are projected onto the STM current image of Fig. 3. (b) Schema of donor sites. Red, blue, and white ellipses represent the charge rich, poor, and unclassified sites, respectively.